1. Field of the Invention
The present invention relates to a solid golf ball having well-balanced properties between repulsion property and shot feeling, irrespective of whether the golf ball is hit at the high head speed or the low head speed.
2. Description of the Related Art
Golf balls are classified into two major groups. The one is the solid golf ball which is excellent in durability and flight distance, and the other is the wound-core golf ball which is excellent in controllability and shot feeling. The former solid golf ball includes a two-piece golf ball comprising a core covered with a cover material, and a multi-layered golf ball having at least one intermediate layer between a core and a cover.
The core of the solid golf ball is usually obtained by vulcanizing-molding a rubber composition which has a formulation of polybutadiene as a base rubber, a metal salt of xcex1, xcex2-unsaturated carboxylic acid and an organic peroxide. The metal salt of xcex1, xcex2-unsaturated carboxylic acid serves as a co-crosslinking agent in the rubber composition and is grafted onto the main chain of the polybutadiene by the organic peroxide serving as a free-radical initiator. Since the rubber composition is molded by vulcanization into the core and forms a three-dimensional cross-linking structure therein, the core exhibits moderate hardness and durability. Further, it is known that the solid golf ball having the above core exhibits superior durability and satisfactory repulsion performance and flying performance.
However, the solid golf ball has a problem that it is relatively hard and gives a relatively large impact (a relatively poor shot feeling) to the golfers when it is hit, compared with the conventional wound-core golf ball. Attempts have been made to lower the hardness of the core in order to improve the shot feeling. However, a resulting golf ball has not exhibited a sufficient flight distance due to the lowered repulsion performance, although the shot feeling has been improved. For example, Japanese Patent No. 2669051 discloses that an organic sulfur compound is blended into the core rubber composition in order to improve the flight distance.
In playing golf, the golfer can choose a golf club and the manner of swinging appropriately depending upon the situation of the golf course, but the golfer cannot change a golf ball throughout all the holes as a general rule. For this reason, a golf ball is required to exhibit stable flying performance and repulsion property and impart a favorable shot feeling at any head speed from low speed to high speed.
However, since the repulsion property and shot feeling of the golf ball vary depending on the kind of the organic sulfur compounds formulated in the rubber composition, a golf ball of which the repulsion property is compatible with the shot feeling at any head speed has not been obtained yet.
The present invention has been achieved in view of such circumstances, and it is an object of the present invention to provide a solid golf ball which exhibits stabilized repulsion performance and gives a superior shot feeling to the golfer when it is hit at any head speed.
According to the present invention, there is provided a solid golf ball comprising a core containing at least one layer, and a cover containing at least one layer and covering the core, wherein
the at least one layer of the core is molded by vulcanizing a rubber composition including: (a) a base rubber, (b) a co-crosslinking agent, (c) an organic peroxide, and (d) a bromine-substituted thiophenol compound represented by the following formula, or a monovalent or divalent metal salt thereof: 
(where at least one of R1 to R5 is Br).